Global navigation satellite systems (GNSS) such as the global positioning system (GPS) are widely used to obtain accurate location information based on the observation of satellite signals. This location information can be used directly for navigation or provided to other devices such as computers or communication terminals to provide enhanced functionality.
A particular challenge upon start-up of a GPS receiver is obtaining a first position fix in a timely manner. This challenge stems from the fact that, given a relatively unknown position, highly accurate time information is required by the GPS receiver in order to locate and interpret satellite signals in a timely manner. In particular, accurate timing information may be required to accurately predict and/or interpret almanac, ephemeris, and navigation signal data and associated signal delays therein indicative of satellite range. However, it is currently not feasible to provide a local clock which can maintain sufficient accuracy for this purpose, particularly between successive activations of the GPS receiver.
U.S. Pat. No. 5,893,044 discloses an apparatus for improving the acquisition time of GPS signals including a GPS receiver and a real-time clock circuit. The GPS receiver receives GPS signals including a precision time reference signal for providing a position based upon the location of the GPS receiver. The GPS receiver also includes an internal time base derived from the precision time reference signal. The real-time clock circuit is coupled to the GPS receiver for receiving a first time reference signal from the GPS receiver when the precise time reference signal of the GPS signal is available and for providing a second time reference signal to the GPS receiver when the precision time reference signal of the GPS signal is not available thereby allowing a fast acquisition time of GPS signals when the GPS signals are temporarily interrupted or not yet available.
United States Patent Application Publication No. US 2007/0268180 discloses a generic navigation satellite system signal receiver having a fast time to first fix by calibrating a low power always-on real time clock (RTC). The receiver includes an RTC calibrator having a fraction calculator and a time expander. Before the receiver is powered off, the fraction calculator uses the fine resolution of a GNSS time signal for determining a time fraction for RTC time. When the receiver is powered back on, the time expander uses an estimate of RTC time drift during the time that GNSS receiver had power off and the time fraction for calibrating and increasing the resolution of the RTC time for an RTC time tick. A signal navigation processor uses the calibrated RTC time for assisting a first fix with code phase search, integration time periods, resolution of epoch integer and/or location-in-space of GPS satellites.
A problem with the above solutions is that they require a dedicated local clock. However, it may not be feasible for some GNSS or GPS-enabled devices to incorporate such a clock. For example, GPS adapters and GPS-enabled wireless adapters for use with a host computer system may not incorporate a dedicated local clock due to constraints such as cost, power, and space.
Another method for obtaining an accurate time signal for a position fix operation is to obtain the time signal from a terrestrial network, for example a wireless network. For example, U.S. Pat. No. 7,236,883 discloses an aided GPS subsystem within a wireless device, the wireless device receiving an external clock signal which is forwarded to the GPS subsystem. However, reliance on an external network can increase cost and complexity of the device, and is subject to network service availability which may not exist in remote locations.
Therefore, there is a need for a new method and apparatus for a global navigation satellite system receiver coupled to a host computer system.